Generally, in transmitting digital information over a wireless communication channel, a data transmitter uses data transformation techniques for transforming a sequence of discrete information symbols representing the digital information into a continuous waveform on a carrier for transmission over the channel to a data receiver. The receiver reverses the transmit process using complementary techniques in order to recover the original information sequence. Often, the transmitter performs a sequence of sub-transformations during the transformation process, depending upon the synthesis approach employed. The synthesis approach, for example, comprises source encoding, error-control encoding, waveform generation, and frequency translation. For present, illustrative purposes, the overall transmitter process will be referred to as "modulation", and the receiver process will be referred to as "demodulation". Two goals of a communications system design is to effect this modulation/demodulation for an intended communications channel with a high throughput of data, i.e., the transmission of data at a data transfer rate as high as possible within the bandwidth limitations imposed by the communications channel, and with an adequately low probability of corruption or loss of the information.
In many applications, the frequency bandwidth within which the data is transmitted is limited by government regulations (such as those imposed in the United States by the Federal Communications Commission, or FCC), so when designing a system emphasis is placed on transmitting data at higher data rates within the allotted band of frequencies. For example, when designing wireless computer networks, such as a local area network (LAN), the FCC currently has allocated about 80 MHZ of bandwidth for local transmission and reception for unlicensed transmitters. If data occupies a relatively small bandwidth of frequencies (e.g., voice data would occupy 20 KHz worth of bandwidth), one can utilize the entire allocated bandwidth to widen the transmitted data (or spread the spectrum of the data) to make the transmission more robust. For some modulation techniques, more bandwidth allows for faster transmission of binary symbol data at faster rates. However, as the data rate of the binary symbols increases, multipath interference becomes more of a problem to the point where the faster data will become difficult if not incomprehensible to understand as the data is received at the receiver. Accordingly, designs often slow down the rate at which data is transmitted so that the data can be understood without concern of multipath interference.
In general, different values of input data to be transmitted can be thought of as respectively representing different symbols with the total number of possible symbols representing a signaling alphabet. Each data input symbol therefore can be represented by a unique waveform which can be transmitted through a transmission medium. By serially transmitting these waveforms (representing a series of data input symbols) one can transmit a message that conveys meaning. The waveform representing each symbol and transmitted through the medium thus must be uniquely recognizable, and must be selected from a set of possible unique waveforms used to represent the possible symbols of the signaling alphabet. The number of possible symbols in a signaling alphabet is referred to as the order of the alphabet. Thus, one approach to increasing data transfer rates is to use a higher-order signaling alphabet to represent the input data with a corresponding number of unique waveforms for transmitting the information through the transmission medium.
A data transmitter capable of robustly transmitting data at high data rates by using a higher-order signaling alphabet is described in our prior application U.S. application Ser. No. 08/369778 entitled a High-Data-Rate Wireless Local-Area Network filed Dec. 30, 1994, and assigned to the present assignee (the "Pending Application"), which application is a continuation-in-part of U.S. application Ser. No. 08/198,138, filed Feb. 17, 1994, now abandoned. The Pending Application describes an improved data transmitter and receiver in which one preferred embodiment uses a direct sequence, spread spectrum (DSSS) signal format when communicating between the two devices. The preferred system uses unique digital waveforms forming a relatively high-order signaling alphabet to communicate between the two devices. These waveforms comprise a set of unique waveforms, which are mutually orthogonal with respect to one another. By using a set of 16 unique waveforms, for example, sixteen different symbols can be represented. Having a choice of one of 16 waveforms thus accounts for 4 binary bits for representing 16 different data input symbols since four bits, or 2.sup.4, has 16 possible values.
The result is to greatly increase the available data rates for wireless LANs while maintaining power efficiency and robustness in the presence of a multipath environment. This can be contrasted to prevailing "digital-radio" techniques employed on telephone microwave trunking links, which use Quadrature Amplitude Modulation (QAM), and which are very bandwidth efficient, but which are very power inefficient and would degrade terribly in multipath typical of LAN environments.
Accordingly, it is desirable to provide faster data rates while maintaining power efficiency with robustness in the presence of a multipath environment.